WARREN. — ALKALI-GRANITES AND PORPHYRIES. 285 



and northern contact, is beautifully exposed and runs in a nearly east- 

 west direction to a point a little northeast of the summit. Up to this 

 point, as along the southern contact, aporhyolite is continuously 

 bordered by the coarsely porphyritic and relatively thin zone of 

 quartz-feldspar- and granite-porphyry (with xenoliths of rhomben- 

 porphyry type) earlier described in detail as characteristic of the 

 deeper contact levels. The immediate contact of the two is often 

 very difficult to make out and thin sections are often rec^uired to really 

 certainly distinguish the two rocks at the contact. As noted, the por- 

 phyry, is at the immediate contact, very dense and feebly porphyritic 

 besides being characterized by breccia and flow structures and gives 

 on megascopic examination alone, the impression that it belongs 

 rather to the aporhyolite than to the porphyry. As can be seen by 

 referring to the special map of this area, the trend of the contact at a 

 point northeast of the summit is irregular, and finally takes a sharp 

 turn to the southwest running along the western base of the hill. 

 The contour of this mass is clearly most irregular. The contact is 

 usually steeply inclined so far as can be told. At one point near the 

 extreme eastern end of the mass it is clearly exposed on a steep cliff 

 marking the western wall of a small valley, and the contact porphyry 

 can here be seen undercutting the aporhyolite with a dip to the south 

 of about 45 degrees. 



Whether the aporhyolite on Pine Hill is actually connected with 

 that within the reservation to the west cannot be told, owing to the 

 rather deep, drift filled valley that intervenes. It is probable, as 

 suggested by Crosby that this valley was once occupied, in large part 

 at least, by slate which was eroded away much more easily than the 

 igneous rocks. Contact with the porphyry is found again south of 

 Rattlesnake Hill and can be traced in a westerly direction across the 

 northern slopes of Wampatuck Hill, thence in a gentle curve around 

 the western top of this hill and back along its steep sonthern face. 

 South of this last line of contact, on the low prominent ledges lying 

 immediately south of the main hill, granite porphyry appears again 

 and extends nearly to the road where it is in contact with the aporhyo- 

 lite again. The low, rounded knolls that occur directly south and 

 southeast of the road at this point, consist in part of quartz-feldspar 

 porphyr;^' or granite-porphyry, and in part of aporhyolite with char- 

 acteristic contacts exposed in several places. These are the "is- 

 lands of quartz-porphyry" referred to by Professor Crosby in his 

 report. Beyond these "islands" to the south, the aporhyolite, so far 

 as can be determined, extends continuously to the borders of the Blue 

 Hill Reservation and probably beyond. 



